Flip chip technology is an alternative to conventional wire bonding technology. Flip chip technology uses face up chips (or dies) and individual wires for connecting each electrical pad on a top surface of a chip to the supporting substrate. Conversely, a flip chip microelectronic assembly enables direct electrical connections between the electronic components of a chip and a corresponding supporting substrate. The electronic components are generally mounted face down (i.e., “flipped”) on the semiconductor chips and connected, both electrically and physically, to the substrate by conductive solder bumps. Flip chips are beneficial because the chip is directly attached (electrically and physically) to the semiconductor substrate, board, or carrier by the solder bumps.
Flip chips may be fabricated by a process that includes placing solder bumps on a semiconductor substrate. The solder bumps of the flip chip assembly provide an electrically conductive path from the chip to the semiconductor substrate on which the chip is mounted. The bumps also provide a thermally conductive path to carry heat from the chip to the semiconductor substrate. The bumps generally provide the mechanical mounting of the chip to the semiconductor substrate. The bumps may also act as a spacer that prevents unwanted electrical contact between the chip and the semiconductor substrate connectors. The bumps can relieve mechanical strain between the chip and the semiconductor substrate.
Unfortunately, fine pitch flip chip technology is limited when providing sufficient and reliable semiconductor substrate pad to die bump joints due to the inability to provide a confined wetting surface for the solder bumps. As described herein, the term “wetting” may refer to the degree with which a liquid maintains adherence to a solid surface for bonding two materials. In this scenario, a wetting surface on the semiconductor substrate enables a connection between the solder bumps on the flip chip and the bond pads or traces on the semiconductor substrate.
One technique for priming a semiconductor substrate for solder ball connection is the application of a flux material. The flux material may be applied on an oxidized portion of a trace/pad for reactivating the trace/pad on a semiconductor substrate to join with a conductive interconnect (e.g., a solder ball). Unfortunately, the application of a flux material can cause voiding in the process due to outgassing of the flux material. Another technique is the application of a surface finish on the trace/pad of the semiconductor substrate to enable the solder ball connection. Unfortunately, the use of a surface finish may activate the entire trace, rather than a specific bond area for joining with a conductive interconnect (e.g., a solder ball). In this scenario, the solder from the solder balls may wet through the entire trace/pad, beyond the specific bond area. Open circuits and/or bridging may result from the solder wetting on the entire trace.